In article <3sg42m$8ji at news0.cybernetics.net>,
"Ronald J. Riegert" <riegerrj at esvax.dnet.dupont.com> wrote:
-> I have recently started working with NDSolve and just
->encountered Mathematica interpolating functions for the first
->time. The Mathematica book says that these can be dealt with
->just like any other function, but I'm having difficulty using
->their derivatives in parametric plots.
-> In particular, I use NDSolve to solve the second-order
->differential equation diffeq for x[t], and call the result
->soln:
->
->soln=
-> NDSolve[{diffeq,x[0.]==0.,x'[0.]==1.},x,{t,0.,50.}][[1,1]]
->
->This gives the interpolating function:
->
->x -> InterpolatingFunction[ {0.,50.}, <>]
->
->I'm interested in plotting D[x[t],t], so I do:
->
->Plot[Evaluate[D[x[t] /. soln,t]],{t,0.,50.}]
->
->which works fine. However, I'm really want to use D[x[t],t]
->in a parametric plot, so I test by entering:
->
->ParametricPlot[{t, Evaluate[D[x[t] /. soln,t]]},{t,0.,50.}]
->
->which should give the same graph as the above Plot command.
->Instead, it produces a bunch of error messages.
-> My question, therefore, is how do I go about using the
->derivative of an interpolating function in a parametric plot?
->
->Ronald J. Riegert
->DuPont Experimental Station
->riegerrj at esvax.dnet.dupont.com
->
->
You need to Evaluate the entire first argument:
ParametricPlot[Evaluate[{t, D[x[t] /. soln,t]}],{t,0.,50.}]
ParametricPlot has attribute HoldAll, so Mma hold the first argument, which
is the part enclosed in braces. Basically, it didn't see your Evaluate
until after substituting 0. for t. I think.
Paul
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